This invention relates to differential global positioning systems (GPS) and methods, and particularly to the enablement and performance of compacted storage and transmission of position-oriented and residual information for use with GPS systems and methods.
Differential GPS systems and methods are generally known. Such systems and methods are summarized in a survey article by Earl G. Blackwell, "Overview of Differential GPS Methods," 32 Journal of The Institute of Navigation, (No.2, Summer 1985). The article describes, among other things, how a local GPS reference receiver (RR) can be employed to eliminate common errors in the GPS navigation solution of other nearby receivers. As is well known, GPS systems permit users equipped with suitable receivers to make accurate position, velocity, and time determinations worldwide with reference to GPS satellites (also referred to as satellite vehicles or SV's) which are in 12 hour (19,000 km) orbits about the earth. Such satellites continuously broadcast their identification, position, and time using specially coded signals. The broadcast information unfortunately contains errors composed of the satellite's clock error, errors in the satellite's broadcast ephemeris data, and certain signal propagation delays.
It is well known according to the prior art for a roving mobile vehicle to receive satellite signals for data processing to develop pseudoranges corresponding to a plurality of satellites and to transmit these pseudoranges to a base station or reference receiver for development of an accurate position determination for the roving mobile vehicle. Four pseudoranges from separate satellites are required to provide a 3D solution, according to well known techniques of GPS, which effectively enables elimination of the receiver's clock error. The location of each satellite is obtained according to the procedures of the prior art, using the satellite's ephemeris message to allow the receiver to calculate its position, which is defined in earth centered, earth fixed coordinates. Differential GPS techniques permit the elimination or reduction of certain errors common to first and second separated receivers. In particular, when two receivers are in the same vicinity and these two receivers use the same four satellites, certain common errors can either be removed entirely or they can at least be substantially eliminated.
FIG. 1b shows a block diagram of a GPS receiver 20 according to the prior art. GPS receiver 20 includes an orbit calculation function 22 based upon a ballpark position input 23, an ephemeris data input 24, and a line-of-sight input 25a from a line-of-sight input source 25. The orbit calculation function 22 produces an output range indication 26. Line-of-sight input source 25 further provides a line-of-sight input 25b to an ATMOS function 28 which produces an ATMOS output indication 29. GPS receiver 20 further includes a summation function 30 which is effective for adding range indication 26 and ATMOS indication 29 to produce a summation output indication 31. GPS receiver 20 further includes a subtraction function 32 which is effective for subtracting the summation output indication 31 from each of a plurality of measured pseudoranges (PRs). The subtraction function 32 produces a pseudorange error indication 33 for each measured pseudorange, which may collectively be referred to as PR error indications. Line-of-sight input source 25 further provides a line-of-sight input 25c to a least squares function 35. GPS receiver 20 includes least squares function 35, and least squares function 25 receives as inputs, PR error indications 33 and line-of-sight input 25c. Least squares function 25 is effective for producing a position error indication 36 for each of the input measured pseudoranges. According to the prior art, least squares function 25 produces position error indications 36 according to the relationship, EQU POS ERROR=[(H.sup.T H).sup.-1 *H.sup.T ]*PRE
where POS ERROR is a particular position error vector corresponding to a selected pseudorange error value; PRE is pseudorange error vector containing pseudorange values for "m" measured satellites; H is a matrix of line of sight and time values for "m" lines of sight, the matrix being in m rows and four columns, the first column comprising the x components for all m rows, the second column comprising the y components for all m rows, the third column comprising the z components for all m rows, and the fourth column comprising all 1's; and each row of the matrix comprising x, y, z coordinates for a particular line of sight for the satellite represented by the corresponding element of the PRE vector, and the last element of each row being the number 1, i.e.: ##EQU1## where m is a selected number of lines of sight greater than or equal to four (4) for purposes of the prior art. GPS receiver 20 further includes a summation function 37 which is effective for adding ballpark (i.e., initial estimated) position indications 23 with corresponding position error indications 36 to produce actual position indications 38, and
PRE.sub.i is the ith pseudorange error, associated with the "ith" line of sight, in the pseudorange vector.
The pseudoranges are useful information specific to particular satellites. In differential GPS, a position correction is established at a base station based upon particular satellites which provide base station specific pseudoranges and a base station specific position correction. To currently determine the position of a rover or mobile unit, the base station can receive current or historical position information from the rover and can further receive pseudoranges established by the rover. The rover pseudoranges were developed based upon a set of satellites which may or may not overlap with the set of satellites used by the base station. If the base station and the rover did not develop their positions and position corrections based upon the same satellites, either the position developed by the rover or the correction developed by the base station are worthless and must be disregarded or thrown away as useless. However, the base station can recalculate its distance correction based upon the pseudoranges provided by the rover. This is generally more economical than asking the rover to redetermine its position determination based upon a new set of satellites. In the recalculation by the base station, only the rover pseudoranges which correspond to the same set of satellites available to the base station will be employed. This entire process accordingly depends upon the transmission of pseudoranges from the rover to the base station. The transmission of these pseudoranges is non-trivial, because of the large bit-size of a typical pseudorange. For example, the table which follows illustrates the numbers of bits which are transmitted and/or stored in several cases involving more or less satellite vehicles. The table follows:
TABLE A ______________________________________ TOTAL NUMBER OF BITS OF NUMBER OF INFORMATION TRANSMITTED OR SATELLITE STORED VEHICLE FIXES PSEUDORANGES (24 BITS/PR) ______________________________________ 4 96 5 120 6 144 7 168 8 192 ______________________________________
It may be advantageous to enable the development of position information and position error information which is of increased accuracy and compactness over information which can be produced by the systems and methods of the prior art.
It may be advantageous to enable the transmission of information which is of increased accuracy and compactness from a roving mobile vehicle to a base station for global positioning purposes.
It may be advantageous to enable the storage of GPS information which is of increased accuracy and compactness at a roving mobile vehicle for subsequent GPS processing and analysis.